I've seen some old PRR documents which refer to "Lines West of Pittsburgh and Erie".
From sometime around 1870 to 1917, the PRR's holdings west of Pittsburgh and Erie were owned and operated by the Pennsylvania Company, a Pennsylvania corporation created just for that purpose. Remember that the original push to build the Pennsylvania Railroad was to funnel traffic to the port of Philadelphia. That, to counter the growing importance of New York, fostered by the Erie Canal and later the New York Central.
According to Schotter's history of the PRR, the PRR wanted to pick up traffic which was going north to the lakes, then east through New York state to the port of New York. One of their first attempts was to "encourage friendly relations" with railroads such as the Pittsburgh, Ft. Wayne and Chicago.
Lo and behold, even though they had assisted the PFW&C in several ways, they found the Ft Wayne was surveying a route of their own into PA, intending to connect to other railroads. At that, PRR bought a pile of PFW&C stock, leased the PFW&C. Now they were in a position to direct the Fort Wayne where to deliver their freight, so as to benefit the PRR and Philadelphia.
Similarly, farther to the south, they acquired an interest in the Pittsburg, Cincinnati, Chicago and St. Louis. Both those were not a single railroad, but an amalgamation of numerous railroads operated under leases. Rather than add to the bookkeeping burden in the PRR's offices in Philadelphia, and perhaps to keep Pennsylvania politicians off their back, they incorporated the Pennsylvania Company to run the western lines.
Eventually, for whatever reasons, PRR decided to transfer the leases and ownership from the Pennsylvania Company to the PRR. However, the term "Lines West" lived on for decades after that.